Cnidaria

                                 Cnidaria is a phylum which has over 9000 species of animals found mainly in oceanic environments.  They are distinguished by a feature called Cnidocytes, which are specialized cells that are mainly used for capturing prey.  Most Cnidarians are constricted to shallow water due to there dependence on endosymbiotic algae for much of their nutrients.  However, some major Cnidarian groups have managed to escape these limitations.

Hydrozoans have a range world wide
          -  Hydra, live in freshwater
          -  Obelia live in the coastal waters of all oceans
          -  Liriope can form large shoals near the surface in mid-ocean.

Among Anthozoans a few sea pens, sea fans and scleractinian corals live in deep chilly waters, some sea anemones inhabit polar seabeds while others live near hydrothermal vents over 10km below the sea level.

http://vimeo.com/23079092
Amazing video of swimming with jellyfish.